For many uses of industrial fabrics, it is critical for such fabrics to possess a controlled and limited air permeability. This is particularly true of fabrics for use in airbags since permeability affects the ability of the bag to inflate in a controlled and consistent manner according to a given design, so as to ensure the effectiveness of the system during a collision event.
Airbag equipment installed on a fixed portion of an automobile, airplane or other passenger vehicle in opposed relation to an occupant therein plays an important roll in the protection against injury to that occupant which may arise due to a collision with a fixed portion of the vehicle body during an accident. As will be appreciated, airbags may be produced in a number of manners and from a number of different materials. However, airbags are typically formed, at least in part, from some type of woven textile material. Such textile materials are disclosed for example in U.S. Pat. No. 5,508,073 to Krummheuer et al. issued Apr. 16, 1996; U.S. Pat. No. 5,503,197 to Bower et al. issued Apr. 2, 1996; U.S. Pat. No. 5,356,680 to Krummheuer et al. issued Oct. 18, 1994; U.S. Pat. No. 5,421,378 to Bower et al. issued Mar. 30, 1994; U.S. Pat. No. 5,277,230 to Sollars, Jr. issued Jan. 11, 1994; U.S. Pat. No. 5,259,645 to Hirabayashi et al. issued Nov. 9, 1993; U.S. Pat. No. 5,110,666 to Menzel, et al. issued May 5, 1992; U.S. Pat. No. 5,093,163 to Krummheauer, et al. issued March 1992; U.S. Pat. No. 5,073,418 to Thornton et al. issued Dec. 17, 1991; U.S. Pat. No. 5,011,183 to Thornton et al. issued Apr. 30, 1991; U.S. Pat. No. 4,977,016 to Thornton et al. issued Dec. 11, 1990; U.S. Pat. No. 4,921,735 to Bloch issued May 1, 1990; and U.S. Pat. No. 3,814,141 to Iribe et al. issued Jun. 4, 1974 (all specifically incorporated herein by reference).
As will be appreciated, very low air permeabilities may be achieved through the use of coatings applied to a fabric construction. However, the use of such coatings presents a disadvantage from both an economic as well as a functional standpoint. Specifically, the use of coatings may add substantial cost to the finished product while at the same time adding bulk to the finished product which translates to a greater folded volume of the final airbag configuration thereby requiring a greater allocation of space within the vehicle deployment system.
In the attempt to avoid the use of coatings while at the same achieving low and controlled air permeabilities, a number of approaches have been taken. The above-referenced U.S. patents to Thornton et al. and Bloch propose the achievement of low permeability characteristics through the use of mechanical deformation processes, in particular calendering, to close the voids at the interstices between overlapping yarns in the fabric so as to reduce permeability to a desired level. While such calendering operations may reduce permeability, this operation may also have the affect of increasing cost while yielding undesirable changes to the flexibility and feel of the fabrics.
In another approach, fabrics have been produced using very tight weave constructions thereby packing the yarns so tightly together that permeability is reduced to a desired level. In some instances, it is necessary to achieve permeabilities in the range of about 1 cubic foot per minute per square foot of fabric or less as measured at a pressure differential of 125 Pa (0.5 inches of water) in order to meet certain automotive specifications. In order to achieve such levels through tight weave constructions, it has generally been thought necessary to pack the yarns at a density to achieve a fabric cover factor of greater than 0.85. One such known construction is a 420 denier nylon fabric having 57 threads per inch in the warp and 53 threads per inch in the fill sold under the trade designation MICROPERM.TM. by Milliken & Company in LaGrange, Ga.
As will be appreciated by those of skill in the art, the term "fabric cover factor" is used to define the packing factor of yarns in a fabric construction in relation to the maximum number of threads which can lay side by side. The maximum number of threads per inch is defined by the following formula:
1. Maximum threads per inch=(fiber specific cover factor).times.(the square root of the cotton count). PA1 2. Cotton count=5315/denier. PA1 3. Warp cover factor=threads per inch in the warp divided by maximum threads per inch. PA1 4. Fill cover factor=threads per inch in the fill divided by maximum threads per inch. PA1 5. Fabric cover factor=(warp cover factor+fill cover factor)-(warp cover factor.times.fill cover factor).
Cotton count is defined by the formula:
The fiber specific cover factors have been determined over the years by researchers. By way of example, the generally accepted cover factor for nylon is 24.4.
As will be appreciated, by using formulas 1 and 2 above, one can readily determine the maximum number of threads per inch which could be placed side by side for a given denier and fiber type. One can then determine a relative cover factor based on the actual weave density in a given fabric by the following relations:
By way of illustration, a 420 denier nylon fabric as described above having 57 threads per inch in the warp and 53 threads per inch in the fill has a warp cover factor of 0.66, a fill cover factor 0.61 and a fabric cover factor of 0.866.
While low permeability fabrics having cover factors greater than 0.85 have been available, the formation thereof has required a relatively high number of threads per inch which necessarily increases the raw material costs. Moreover, it is generally understood that in order to achieve such very high cover factors, weaving must be carried out by mechanical (i.e. Rapier) weaving processes rather than by water-jet insertion or other means which are substantially faster and less expensive.
Notwithstanding the achievements which have been made in reducing air permeability to this point, it is anticipated that fabrics of even lower permeabilities achieved without the need for expensive and complex calendering operations or high cover factor configurations represent a desirable and useful advancement over the present art. The fabrics of the present invention may be woven on efficient water-jet weaving machines due to their relatively low cover factor requirement and exhibit desirable low permeability characteristics without requiring the need for complex coatings or calendering operations and thereby represent a useful advancement over the present art.